[1.9.x] Refs #23386 -- Documented that F() expressions are applied on each model.save()

the database when the :meth:`~Model.save()` or ``update()`` is executed, rather

than based on its value when the instance was retrieved.

``F()`` assignments persist after ``Model.save()``

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

``F()`` objects assigned to model fields persist after saving the model

instance and will be applied on each :meth:`~Model.save()`. For example::

    reporter = Reporters.objects.get(name='Tintin')

    reporter.stories_filed = F('stories_filed') + 1

    reporter.save()

    reporter.name = 'Tintin Jr.'

    reporter.save()

``stories_filed`` will be updated twice in this case. If it's initially ``1``,

the final value will be ``3``.

Using ``F()`` in filters

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